There is an integer overflow when allocating memory in the routine that handles
loading PNG image files. This later leads to heap data structures being
overwritten. If an attacker tricked a user into loading a malicious PNG image,
they could leverage this into executing arbitrary code in the context of the
user opening image. Many programs use GD, such as ImageMagick, and more
importantly it is also the image library used for PHP, and there is a Perl
module as well. One possibile target would be PHP driven photo websites that
let users upload images. Some of them will resize/compress the image when the
user uploads them. If this is done using GD, this could be used to execute code
on the server. There is a mitigating factor, in order to reach the vulnerable
code, a large amount of memory needs to be allocated. My 128MB p2 crapped out
one allocation before it reached the overflow. However, I think on a newer box
with lots of memory and swap space, that won't be a problem.

The vulnerable code occurs in the file gd_png.c, in the function
gdImageCreateFromPngCtx(), which is called by gdImageCreateFromPng(). The
function is used to load an image file into GD data structures. The problem
occurs when allocating memory for the image rows, line 314 or so ( I added some
comments so line number might be off). Two user supplied values are multiplied
together (rowbytes * height), and used to allocate memory for an array of
pointers. This pointer array is then passed to the png_read_image() function,
which belongs to the libPNG library. In that function, the pointers are passed
to the png_read_row() function. The data for the rows is decompressed using
zLib function inflate(), and then passed to the png_combine_row() function,
where the deflated data is memcpy()'d into the heap buffer. Exploitation would
require using zLib functions to compress the payload. Successful exploitation
would lead to executing arbitrary code.